Spatial Light Modulators (SLMs) or light valves are widely used in the industry for video monitors, graphic displays, projectors, and hard copy printers. SLMs and light valves are devices that modulate incident light in a spatial pattern corresponding to an electrical or optical input. The incident light may be modulated in its phase, intensity, polarization, or direction. This light image is directed and focused to a screen in the case of a projector, video monitor or display, or is ultimately focused on a light sensitive material, such as a photoreceptor drum, in the case of a xerographic printer.
The light modulation may be achieved by a variety of materials exhibiting various electro-optic or magneto-optic effects, and by materials that modulate light by surface deformation. Other spatial light modulators may include tiny micro-mechanical devices comprising an array of positionable picture elements (pixels). The light image can be colored if it is to be displayed on a screen of a projector, monitor, or a television and the like. This coloring is typically done in one of two ways, either using non-sequential color systems, or using sequential color systems. A non-sequential color system simultaneously images multiple colors of light, such as red, green and blue light. An example of a non-sequential color system is discussed in commonly assigned U.S. Pat. No. 5,452,024, to Sampsell, entitled "DMD Display System", the teachings included herein by reference. In sequential color systems, color images are generated by sequentially projecting imaged colored light, i.e. red, green and blue light, in a single image frame, which typically lasts 1/60 of a second. Sequential color systems typically utilize a color wheel that is partitioned into a plurality of color of segments, such as a red, green, and blue segment, or multiples/combinations thereof. An example of a sequential color system is disclosed in commonly assigned U.S. Pat. No. 5,448,314 to Heimbuch, et al entitled "Method and Apparatus for Sequential Color Imaging", the teachings included herein by reference.
A recent innovation of Texas Instruments Inc. of Dallas, Tex. is an SLM imaging system using an array of individual micro-mechanical elements, known as a digital micromirror device (DMD), also referred to as a deformable mirror device. The DMD is a spatial light modulator suitable for use in displays, projectors and hard copy printers. The DMD is a monolithic single-chip integrated circuit, comprised of a high density array of 17 micron square deflectable micromirrors. These mirrors are fabricated over address circuitry including an array of SRAM cells and address electrodes. Each mirror forms one pixel of the DMD array and is bi-stable, that is to say, stable in one of two positions. A source of light directed upon the mirror array will be reflected in one of two directions by each mirror. In one stable "on" mirror position, incident light to that mirror will be reflected to a collector lens and focused on a display screen or a photosensitive element of a printer, and forms an image of the mirror/pixel. In the other "off" mirror position, light directed on the mirror will be deflected to a light absorber. Each mirror of the array is individually controlled to either direct incident light into the collector lens, or to the light absorber. In the case of a display, a projector lens and a light prism ultimately focus and magnify the modulated light image from the pixel mirrors onto a display screen and produce a viewable image. If each pixel mirror of the DMD array is in the "on" position, the displayed image will be an array of bright pixels.
For a more detailed discussion of the DMD device, cross reference is made to U.S. Pat. No. 5,061,049 to Hornbeck, entitled "Spatial Light Modulator and Method"; U.S. Pat. No. 5,079,544 to DeMond, et al, entitled "Standard Independent Digitized Video System"; and U.S. Pat. No. 5,105,369 to Nelson, entitled "Printing System Exposure Module Alignment Method and Apparatus of Manufacture", each patent being assigned to the same assignee of the present invention and the teachings of each are incorporated herein by reference. Gray scale of the pixels forming the image can be achieved by pulse width modulation techniques of the mirrors, such as that described in U.S. Pat. No. 5,278,652, entitled "DMD Architecture and Timing for Use in a Pulse-Width Modulated Display System", assigned to the same assignee of the present invention, and the teachings of which are incorporated herein by reference.
In non-sequential color systems, three (3) DMD arrays can be used to form an image at an image plane, one DMD for modulating red, green, and blue light, as disclosed in the commonly assigned U.S. Pat. No. 5,452,024, to Sampsell, titled "DMD Display System", the teachings of which are included herein by reference. In contrast, a sequential color system requires only one such DMD device, with the red, green, and blue light being sequentially modulated and reflected by the single DMD array to an image plane . The need for three such DMD arrays in the non-sequential color system triples the requirement for the DMD arrays, and attendant hardware over the sequential color system, but offers increased display brightness. Thus, there is a trade off between the complexity, cost and performance of a non-sequential color system when viewed against a single DMD sequential color system.
In the case of a sequential color system, a single light source is typically used, such as disclosed in U.S. Pat. No. 5,101,236 to Nelson, et al, entitled "Light Energy Control System and Method of Operation", assigned to the same assignee as the present invention and the teachings of which are included herein by reference. The lamp may typically be comprised of a xenon or metal halide arc lamp, or a laser. This arc lamp may be powered by an AC or DC power source.
Multiple light sources can also be implemented in a sequential color system using a single light valve, as disclosed in U.S. Pat. No. 5,428,408. This system includes three projection lamps, one for each of the primary colors, which are sequentially activated. Three occluders are utilized, one blocking or unblocking the light output from the associated lamp. The light output from the associated lamp that ultimately illuminates the light valve is controlled by the pulse driven occluders.
Conventional arc lamps, which may consist of xenon or metal halide arc lamps, are typically deficient in intensity in some portion of the color spectrum. That is, for a given power input, the associated light output levels of red, blue, and green light are unbalanced. A typical lamp is most deficient in red light, and most sufficient in green light. One solution is to address color balance disclosed in the commonly assigned U.S. patent application Ser. No. 08/414,707 entitled "Spatial Light Image Display System with Synchronized and Modulated Light Source", where each of three lights can be individually driven and amplitude modulated to achieve color balance. The teachings of this patent application are included herein by reference.
It is desired to provide a sequential color imaging system utilizing a single light valve or spatial light modulator, and only two lamps to provide a cost efficient color balanced and bright system. It is further desired to implement conventional arc lamps that require operation at a rated power level in order to provide optimal power dissipation.